1. Field of the Invention
The present invention relates to a light beam scanning device which two-dimensionally scans an object region with a light beam such as a laser beam.
2. Related Art
As a device such as a vehicle gap measuring apparatus, known is an apparatus configured by: a light beam scanning device which two-dimensionally scans a certain measuring object region in front of the apparatus with a laser beam; a light receiving device which receives reflected light of the laser beam emitted from the light beam scanning device; and a calculation device which calculates a vehicle gap on the basis of a light receiving state in the light receiving device. For example, Unexamined Japanese Patent Publication Hei.9-274076 discloses such a vehicle gap measuring apparatus.
A light beam scanning device of the conventional art which is used in a vehicle gap measuring apparatus is structured in the following manner. Pulse light from a light source such as a laser diode is passed through a collimating lens to be emitted as a light beam. The light beam impinges on light deflecting means which is swung in a constant angle range, specifically a reflector plate or a rotating polygon mirror. A measuring object region in front of the apparatus is two-dimensionally scanned by using a reflected beam from the means. In the vehicle gap measuring apparatus disclosed in the patent publication, for example, two-dimensional scanning of a light beam is conducted by using a polygon mirror.
As described above, in a light beam scanning device of the prior art, a light beam is deflected by swinging a reflector plate or rotating a polygon mirror, so as to conduct a scanning operation in a constant angle range. Deflecting means such as a reflector plate requires a larger installation space than other components. This largely impedes miniaturization of a light beam scanning device.
In a light beam scanning device of the prior art, irradiation of a light beam is pulsatingly conducted in synchronization with the scanning angle. When the irradiation (emission) timing of a light beam is shifted from the rotational angle position (scanning angle) of a reflector plate or a polygon mirror, there arises a disadvantage that it is impossible to scan the whole of a measuring object region without omission. In order to prevent such a disadvantage from occurring, conventionally, the reflector plate or the polygon mirror is driven while maintaining accurate linearity with respect to the irradiation timing of the light beam. Therefore, a mechanism for driving the reflector plate or the polygon mirror must be provided with positional accuracy of a high resolution.